Silver halide dispersions



b8... IWJ), A

United States Patent M SILVER HALIDE DISPERSIONS Forrest V. Williams,Dayton, Ohio, assignor to Monsanto Chemical Company, St. Louis, Mo., acorporation of Delaware No Drawing. Filed Dec. 18, 1957, Ser. No.703,502

11 Claims. (01. 96-114 The present invention relates to dispersion ofsilver halides and more particularly provides aqueous dispersions ofsilver bromide of very fine grain size and increased sensitivity inphotographic emulsions comprising the dispersions.

It is well known in the art to prepare silver halides for photographicpurposes by effecting the double decomposition reaction leading theretoin the presence of a peptizing or dispersing agent in order to obtainfine grain size of the precipitated silver holide; and in prior art,there have been employed to this end such dispersing agents as cellulosemethyl ether, diethanolamine cellulose acetate, polyvinyl pyridine, gumarabic, and other watersoluble natural resins, etc. General purposedispersants, particularly thosecontaining sulfonate and/or carboxylategroups, could be employed only with caution owing to the possibility ofconversion to insoluble silver salts thereof. Obviously, also, therecould not be employed as dispersant an agent which would react with theother customarily employed components of photographic emulsrons.

Now I have found that very fine grain size and improved sensitivity ofphotographic silver halides can be attained by preparing the halides inthe presence of a dispersing agent comprising an aliphaticmono-olefinmaleic anhydride copolymer consisting of the repeating unit.

wherein Z is a bivalent alkylene radical of from 2 to 4 carbon atoms andX and Y are selected from the class consisting of the radicals -OH, ONH--NH and alkali metal.

One class of presently useful copolymers includes salts of maleic acidaC-C; aliphatic, mono-olefin copolymers with ammonia or an alkali metal,e.g., the ammonium, potassium, sodium or lithium salts ofethylene-maleic acid or ethylene-isobutylene-maleic acid copolymer.

Another class of presently useful copolymers includes the monoanddiamides of the C C alkene-maleic acid copolymers. This class ofcopolymer consists of the repeating unit in which Z is as herein definedand T is selected from the class consisting of NH and OH. Asillustrative of copolymers having the above formula may be mentioned themono-amide of maleic acid-ethylene, propylene or 1- or 2-butenecopolymer, the diamide of maleic acid- -ethylene or Z-butene copolymer,etc.

Still another class of the presentlyuseful derivatives of maleic acid-C0 aliphatic mono-olefin copolymers inl atented Oct. 25, 1960acid-ethylene or propylene copolymer mono-amide, and the ammonium saltof maleic acid-lor Z-butene copoly- Iner.

A particularly useful class comprises the maleic acid- C C aliphaticmono-olefin copolymers having free carboxy radicals, i.e., copolymersconsisting of the repeating unit in which Z is as herein defined.

The above presently useful free acids, salts, amides, andhalf-salts-half-amides of maleic acid-C -C aliphatic, mono-olefincopolymers are known materials which are obtainable in commerce or bymethods well known to those skilled in the art. For convenience,however, a resume of such methods is given herewith.

In practice, the presently employed derivatives of maleic acid-olefincopolymers are prepared from readily available maleic anhydride-olefincopolymers. Generally, these copolymers are prepared by reactingethylene, propylene, 1- or 2-butene or mixtures of these ole-fins withmaleic anhydride in the presence of a peroxide catalyst in an aliphaticor aromatic hydrocarbon which is a solvent for the monomers but is anon-solvent for the interpolymer formed. Suitable solvents includebenzene, toluene, xylene, chlorinated benzene, hexane, acetone and thelike. While benzoyl peroxide is the preferred catalyst, other peroxidessuch as acetyl peroxide, butyryl peroxide, ditert-butyl peroxide,lauroyl peroxide and the like are all satisfactory since they aresoluble in organic solvents. The copolymer contains substantiallyequirnolar quantities of the ethylene residue and the maleic anhydrideresidue. The properties of the copolymer such as molecular weight, forexample, may be regulated by proper choice of the catalyst and controlof one or more of the variables such as ratio of reactants, temperature,and catalyst concentration. For the present purpose, the salts ofhalf-amides, half-salts or diarnides of the C O, aliphaticmono-olefinmaleic acid copolymers prepared from the correspondingalkene-maleic anhydride copolymers are generally useful, i.e., eitherthe high or low-molecular weight copolymers are useful. Thus, the degreeof polymerization may be from, say, 2 to 2,000, corresponding to anaverage molecu lar weight of, say, 250 to 250,000. Particularly usefulare copolymers having an average molecular weight of, say, 600 to200,000. The anhydride copolymer product is obtained in solid form andis easily recovered by filtration, centrifugation or the like. Removalof any residual or adherent solvent may be elfected by evaporation usingmoderate heating.

The maleic anhydride copolymers thus obtained have The monoor diammoniumor alkali metal salts may be readily obtained by reacting the copolymerin its anhydride or acid form with the stoichiornetric amount ofammonium hydroxide or alkali metal hydroxide. Mixed salts, e.g.,half-ammonium, half-alkali metal salts are prepared by first reactingwith a quantity of ammonium hydroxide calculated to give the partialammonium salt and then reacting the residual free carboxy radical withsufficient alkali metal hydroxide to neutralize said radical.

Amides are prepared generally by reacting the finely divided maleicanhydride-C C; olefin copolymer with ammonia gas at ordinary or elevatedtemperatures. Halfsalts, half-amides are formed by operating in thepresence of the stoichiometrically required quantity of moisture. Heatis generally liberated in the preparation of the halfammonium salt,half-amide, and it is thus desirable to provide some means fordissipating it so that the product will not be effected by excessivelyhigh temperatures. One effective means for controlling the heat orreaction consists of suspending the solid polymer in an inert organicliquid such as benzene and bubbling ammonia through the slurry.

In preparing the presently provided fine grain, highly photo sensitivedispersions of silver bromide, I operate substantially as follows:

I prepare an aqueous solution of the copolymer compound and awater-soluble bromide in known concentrations and gradually add theretoan aqueous solution of a silver salt with agitation or stirring.Alternatively, I may first prepare an aqueous solution of the copolymerand simultaneously add thereto the water-soluble halide and silver salt.The concentration of the copolymer in the aqueous solution isimmaterial; however, sufficient quantity of the copolymer should bepresent in the reaction mixture to assure dispersion of the silverhalide as it is formed. Because the present copolymers displaypronounced silver halide dispersing activity at very smallconcentrations, advantageously, very dilute aqueous solutions of thecopolymer are employed. The concentration of the copolymer in theaqueous dispersion of the silver halide thus prepared is in the orderof, say, from 0.001% to 0.1% of copolymer based on the total weight ofthe dispersion. Employing the copolymer in such. concentrations, thereare obtained crystals of the silver halide which are of considerablyless than one micron, c.g., from 0.1- 0.5 micron. The dispersions orsuspensions of silver halide thus obtained have pronounced stability, ascompared to those obtained in the absence of the copolymer. Thus whilethe settling time of silver bromide suspensions obtained in the absenceof a dispersant is generally from, say, only a few minutes to an hour,sedimentation in silver halide dispersions prepared according to thepresent process is not evident until a considerably longer length oftime has elapsed and complete settling of the precipitated halide doesnot occur until a time of from, say, several days to several weeks.

The effect of the presently employed copolymer dispersant on the lightsensitivity of silver halides prepared in the presence of thesedispersants is readily noted in experiments wherein silver halideformation is effected in the presence of daylight or ultraviolet light.In dilute solutions of the silver halide, the effect of the copolymer onthe light sensitivity is revealed by pink to violet-brown discolorationsof the microscopic particles within a few minutes and the discolorationdeepens with time without readily visible precipitation of the silverbromide. When the salt ions are more concentrated, the silver halide isvisibly dispersed as it is formed and the dispersions thus obtainedturn. black within one or two minutes. On the other hand, silver halideswhich are obtained by use of the same ion concentrations but in theabsence of the copolymer dispersant, remain substantially colorless forseveral hours.

Aqueous dispersions prepared substantially as above described in thepresence of the C -C aliphatic olefinmaleic acid copolymer compounds aregenerally useful in the preparation of photographic emulsions. As theprotective carrier component of the emulsion there may be employedeither gelatin or any of the customarily used substantiallywater-soluble synthetic resinous materials such as polyvinyl alcohol,partially hydrolyzed polyvinyl acetals or esters, low viscositypolyvinyl esters, etc. The emulsions, prepared in accordance with theinvention, upon application to a supporting material such as film orpaper, give a photographic product which is characterized by highsensitivity and very fine grain structure.

The invention is further illustrated, but not limited, by the followingexamples:

Example 1 The half-amide half-salt of an isobutylene-maleic acidcopolymer (prepared by ammoniating a maleic acid-isobutylene copolymerhaving an average molecular Weight of about 150,000) was dissolved insufficient water to give an 0.025 N solution thereof. It was furtherdiluted to give 0.0125 N and 0.0063 N solutions of said halfamidehalf-salt. The 0.0125 N solution was mixed with 5 ml. of 0.1 N aqueoussodium bromide, and 5 ml. of 0.1 N aqueous silver nitrate was graduallyadded, with stirring, to the resulting mixture. To 2.5 ml. of the 0.0063N solution of copolymer, there was added 7.5 ml. of water and 5 ml. ofthe 0.1 N aqueous sodium bromide, and the resulting mixture was treatedwith 5 ml. of 0.1 N aqueous silver nitrate, also gradually and withstirring. A blank was prepared by adding 5 ml. of 0.1 N aqueous sodiumbromide to 10 ml. of water and gradually adding thereto, with stirring,5 ml. of 0.1 N silver nitrate. Upon addition of the silver nitrate, allof the mixtures became cloudy with suspended silver bromide; but thatprepared in the absence of the copolymer half-amide half-salt began tosettle almost immediately with complete sedimentation Within an hour,Whereas the other two dispersions of the silver bromide remained atleast partially suspended at the end of 48 hours, and completesedimentation had not occurred even at the end of 6 days. Inspection ofthe silver bromide particles present in the dispersions, disclosed aparticle size of over two microns in the case of the blank, whereas aparticle size of less than one micron was determined for the silverbromide crystals obtained in the presence of the copolymer dispersant.

Example 2 An ethylene-maleic anhydride copolymer having a specificviscosity of 0.09 (as determined for a 1% by weight solution thereof indimethylformamide) was mixed with 500 ml. of distilled water to give anethylene-maleic acid copolymer solution having an 0.025 N concentrationof free acid. Dispersions of silver bromide were prepared by graduallyadding, with stirring, 5 ml. of 0.1 N silver nitrate to a mixtureconsisting of 5 ml. of the 0.025 N copolymer solution, 5 ml. of 0.1 Nsodium bromide and 5 ml. of Water for the preparation of one dispersion;and, for the preparation of another dispersion, by gradually adding,with stirring, 5 ml. of 0.1 N silver nitrate to a mixture consisting of10 ml. of the 0.025 N copolymer solution and 5 ml. of the 0.1 N aqueoussodium bromide. The resulting aqueous dispersions of the silver bromidewere still cloudy at the end of 16 hours and complete settling was notobserved even after 5 days.

Example 3 Themaleic anhydride copolymer of Example 2 was converted tothe sodium salt by treating it with aqueous sodium hydroxide to obtainan 0.025 N aqueous solution ofthe sodium salt of the ethylene-maleicanhydride copolymer.

Silver bromide dispersions were prepared by adding 5 ml. of 0.1 N silvernitrate to a mixture consisting of 5 ml. of the 0.025 N copolymer saltsolution, 5 ml. of 0.1 N aqueous sodium bromide and 5 ml. of water inone instance; and, in another instance, by gradually adding, withstirring, 5 m1. of 0.1 N silver nitrate to a mixture consisting of 10ml. of the 0.025 N copolymer salt solution and 5 ml. of 0.1 N sodiumbromide. The aqueous dispersions of silver bromide thus obtained were ofmarked stability, the settling time thereof being several hours ratherthan a few minutes as was the case with dispersions prepared from thesame quantities of sodium bromide, silver nitrate and water in theabsence of the copolymer dispersant.

Example 4 thereof in dimethylformamide.

0.1 N 0.1 N H O, Expt. No. NaBr, AgNO ml. Copolymer ml. m1.

5 5 10 ml. 0.05 N of A. 5 0 ml. 0.005 N of B. 5 5 4 6 ml. 0.005 N MA. 55 4 6 ml. 0.005 N of B. 5 5 8 2 ml. 0.005 N of A. 5 5 8 2ml.0.005NofB. 55 10 None.

5 5 8 2 ml 0.0005 N of A In the above table, the normality of thecopolymer solution is calculated as the free acid. The respectiveexperiments were conducted by adding the silver nitrate to a mixture ofthe sodium bromide and copolymer solution in the presence or absence ofadditional water as indicated. In Experiment VII, wherein no copolymerwas present, complete settling-out of the silver bromide within one hourwas noted. In Experiment VIII, wherein a concentration of about 30p.p.m. of the copolymer, based on the total weight of the dispersion,was employed, the settling-out time was considerably prolonged but didnot attain the time noted in the other experiments, which time generallywas from several hours to a day. The molecular weight of theethylene-maleic anhydride copolymer thus has no marked effect on thedispersing effect of the free acid obtained therefrom although stabilityof the suspensions was found to be a function of the quantity of thecopolymer dispersant employed, dispersing effect is noted when thecopolymer is present in only very small proportions.

Example 5 This example shows the effect on light sensitivity of thehalf-amide half-ammonium salt of isobutylene-maleic anhydride copolymerhalf-amide half-salt of Example 1 when employed in the preparation ofsilver bromide in daylight.

To a mixture consisting of 5 ml. of an 0.025 N aqueous solution of thehalf-amide half-salt, 5 ml. of 0.005 N sodium bromide and 5 ml. ofwater, there was gradually added, with stirring, 5 ml. of 0.005 Naqueous silver nitrate. A "blank was also prepared by adding 5 ml. of0.005 N silver nitrate to a mixture consisting of 5 ml. 0f0.005 Naqueous sodium bromide and 10 m1. of water.

The reaction mixture obtained in the presence of the copolymerhalf-amide half-salt became pink within an hour, whereas the blankshowed no discoloration.

In another set of experiments, wherein an excess of silver nitrate wasemployed, 6 m1. of 0.005 N aqueous silver nitrate was gradually added,with stirring, to a mixture consisting of 5 ml. of the 0.025 N aqueouscopolymer half-amide half-salt, 5 ml. of 0.05 N sodium bromide and 4 ml.of water. A blank was prepared by adding 6 ml. of silver nitrate of thesame normality to a mixture consisting of 5 ml. of the same normality ofsodium bromide and 9 ml. of water. The reaction mixture which wasobtained in the presence of the copolymer half-amide half-salt turnedpink within about 30 minutes, whereas no discoloration was evidenced inthe case of the blank.

In still another set of experiments, the effect of the use of moreconcentrated silver nitrate and sodium bromide was studied. To a mixtureconsisting of 5 ml. of 0.025 N aqueous solution of said copolymerhalfamide half-salt, 5 ml. of 0.1 N aqueous sodium bromide and 4 ml. ofwater, there was gradually added, with stirring, 6 ml. of 0.1 N silvernitrate. A blank was prepared by adding 6 ml. of silver nitrate of thesame normality to a mixture consisting of 5 ml. of the same normality ofsodium bromide and 9 ml. of water. The reaction mixture which wasprepared in the presence of the copolymer half-amide half-salt turned areddishpurple at the end of about 10 minutes, whereas the blank, of amilky, chalky color, showed no discoloration.

Even more highly concentrated solutions of sodium bromide and silvernitrate were employed in a subsequent experiment. To 5 ml. of the 0.025N aqueous copolymer half-amide half-salt, 5 ml. of 2 N aqueous sodiumbromide and 5 ml. of water, there was gradually added, with stirring, 5ml. of 2 N aqueous silver nitrate; and a blank was prepared by adding 5ml. of 2 N aqueous silver nitrate to a mixture consisting of 5 ml. of 2N aqueous sodium bromide and 10 ml. of water. Within a few minutes thereaction mixture which was prepared in the presence of the copolymerhalf-amide half-salt began to turn black, whereas no color change wasperceptible in the blank.

Example 6 An 0.025 N aqueous solution of the sodium salt of Example 7This example shows the effect of ethylenc-maleic anhydride copolymer onthe light sensitivity of silver bromide prepared in the presencethereof. An ethylenemaleic anhydride copolymer having a specificviscosity of 1.11 as determined for a 1.0% solution thereof indimethylformamide, was added to distilled water in a quantity calculatedto give an 0.025 N aqueous solution of the free acid. To a mixtureconsisting of 5 ml. of said free acid solution, 5 ml. of water and 5 ml.of 2 N aqueous sodium bromide, there was gradually added, with stirring,5 ml. of 2 N aqueous silver nitrate. Darkening of the resulting reactionmixture was observed within 5 minutes.

Example 8 This example shows the effect on light sensitivity of silverbromide prepared in the presence of the half-amide half-salt ofethylene-maleic anhydride copolymer prepared by ammoniating anethylene-maleic anhydride copolymer having a specific viscosity of 0.1as determined for an 0.10% by weight solution thereof indimethylformamide. To a mixture consisting of ml. of an 0.025 N aqueoussolution of the half-amide half-salt, 5 ml. of 2 N aqueous sodiumbromide and 5 ml. of water, there was gradually added, with stirring, 5ml. of 2 N aqueous silver nitrate. The resulting reaction mixture beganto darken within 5 minutes and was black at the end of 50 minutes.

Example 9 To a mixture consisting of 20 ml. of 0,025 N aqueous solutionof the half-amide, half-ammonium salt of isobutylene-maleic acidcopolymer described in Example 1, 19.5 ml. of 2 N aqueous potassiumbromide, 0.5 ml. of 2 N aqueous potassium iodide and 20 ml. of water,there was gradually added, with vigorous stirring, 20 ml. of 2 N aqueoussilver nitrate. The resulting silver halide dispersion was mixed with100 ml. of a 10% aqueous solution of polyvinyl alcohol. The whole wasthen diluted with water to give a composition having a viscositysuitable for smooth application to paper, and subsequently sensitizedwith 2,3-diethyl-4-methyloxathiazolocarbocyanine iodide. Photographicpaper prepared from the resulting emulsion by applying it to paper anddrying gave a readily processed product.

As will be apparent to those skilled in the art, all of the aboveexperiments, except those in which the effect of daylight was studied(Examples 5-8) were carried out in the dark in order to avoiddeterioration to the silver halide.

The present C C aliphatic olefin-maleic acid copolymers and amides andsalts thereof are also efiective in preparing aqueous dispersions ofother silver halides, i.e., silver chloride and silver iodide; and, aswith silver bromide there are thereby obtained similarly fine grainproducts having improved light-sensitivity. For numerous purposesaqueous dispersions of mixtures of the silver halide are desirable.These can be prepared, according to the invention, by precipitating amixture of halides in water solution by treatment with a silver salt inpresence of present copolymer dispersants. Thus, aqueous solutions ofsubstantially equal quantities of, say, potassium bromide and potassiumchloride and a minor amount of potassium iodide when treated withaqueous silver nitrate in the presence of, say, the half-amide, halfammonium salt of ethylene-maleic anhydride copolymer yields a dispersionof a mixture of very fine grained silver halides of remarkablephotosensitivity. Such dispersions are readily sensitized by the usualsensitizing dyes, e.g., the cyanine dyes.

As will be apparent to those skilled in the art, many variations may bemade in preparing the present disper- :sions. For example, instead ofadding the solution of :silver salt to the mixture of water-solublehalide, copolymer dispersant and water, the dispersant may be introducedinto the solution of silver salt, and the resulting mixture added to theaqueous solution of halide. Or, if

desired, aqueous solutions of halide may be added either simultaneouslyor alternatively to Water containing the copolymer dispersant. Formationof the water-insoluble silver salt can also be conducted in the presenceof the protective carrier therefor which it is desired to employ as aconstituent of the photographic emulsion. Here the aqueous solution ofsilver salt is added to an aqueous mixture consisting of the presentcopolymer dispersant, water-soluble halide and the carrier, e.g.,polyvinyl alcohol, gelatin, partially hydrolyzed polyvinyl acetate orpolyvinyl butyral, cellulose nitrate, etc. If not water soluble, thecarrier is employed as a solution in a watermiscible solvent.

What I claim is:

1. A photographic emulsion comprising a silver halide of small particlesize prepared by a double decomposition reaction in water between awater-soluble halide and a water-soluble silver salt in the presence offrom 0.001% to 0.1% based on the weight of the total reaction mixture ofa dispersant which is a copolymer consisting of the repeating unitwherein Z is a bivalent alkylene radical of from 2 to 4 carbon atoms andX and Y are selected from the class consisting of the radicals -OH, ONHNH and alkali metal, and a protective carrier for said silver halidewhich is selected from the class consisting of gelatin, polyvinylalcohol, water soluble partially hydrolyzed polyvinyl acetals andesters, and water-soluble low viscosity polyvinyl esters.

2. A photographic emulsion comprising a silver halide of small particlesize prepared by a double decomposition reaction in water between awater-soluble halide and a water-soluble silver salt in the presence offrom 0.001% to 0.1% based on the total weight of the reaction mix tureof a dispersant which is a copolymer consisting of the repeating unitwherein Z is a bivalent alkylene radical of from 2 to 4 carbon atoms anda protective carrier for the silver halide which is selected from theclass consisting of gelatin, polyvinyl alcohol, water-sol-uble partiallyhydrolyzed polyvinyl acetals and esters, and water-soluble low viscositypolyvinyl esters.

3. A photographic emulsion comprising a silver halide of small particlesize prepared by a double decomposition reaction in water between awater-soluble halide and a water-soluble silver salt in the presence offrom 0.001% to 0.1% based on the total weight of the reaction mixture ofa dispersant which is a copolymer consisting of the repeating unit WhereZ is a bivalent alkylene radical of from 2 to 4 carbon atoms, and aprotective carrier for the silver halide which is selected from theclass consisting of gelatin, polyvinyl alcohol, water-soluble partiallyhydrolyzed polyvinyl acetals and esters, and water-soluble low viscositypolyvinyl esters.

4. A photographic emulsion comprising a silver bromide of small particlesize prepared by a double decomposition reaction in water between sodiumbromide and silver nitrate in the presence of from 0.001% to 0.1% basedon the weight of the total reaction mixture of a dispersant which is acopolymer consisting of the repeating unit position reaction in Waterbetween sodium bromide and silver nitrate in the presence of from 0.001%to 0.1%

9 based on the weight of the total reaction mixture of anethylene-maleic acid copolymer as a dispersant, and gelatin as aprotective carrier for the silver bromide.

6. A photographic emulsion comprising a silver bromide of small particlesize prepared by a double decomposition reaction in water between sodiumbromide and silver nitrate in the presence of from 0.001% to 0.1% basedon the weight of the total reaction mixture of the half-amide,half-ammonium salt of isobutylene-maleic acid copolymer, and gelatin asa protective carrier for the silver bromide.

7. The process of preparing a photographic emulsion which comprisesfirst preparing an aqueous dispersion of a silver halide of smallparticle size by conducting in water a double decomposition reactionbetween a water soluble halide and a water-soluble silver salt in thepresence of from 0.001% to 0.1%, based on the weight of the totalreaction mixture, of a dispersant which is a copolymer consisting of therepeating unit Z--CH-CH =0 l: t wherein Z is a bivalent alkylene radicalof from 2 to 4 carbon atoms and X and Y are selected from the classconsisting of the radicals OH, ONH NH; and alkali metal, andsubsequently adding thereto, as a protective carrier for the silverhalide, a polymeric mate rial selected from the class consisting ofgelatin, polyvinyl alcohol, water-soluble partially hydrolyzed polyvinylacetals and esters, and Water-soluble low viscosity polyvinyl esters.

8. The process of preparing a photographic emulsion which comprisespreparing an aqueous dispersion of a silver halide of small particlesize by conducting in water a double decomposition reaction between awatersoluble halide and a water-soluble silver salt in the presence offrom 0.001% to 0.1%, based on the weight of the total reaction mixture,of a dispersant which is a copolymer consisting of the repeating unitwherein Z is a bivalent alkylene radical of from 2 to 4 carbon atoms,and subsequently adding thereto, as a protective carrier for the silverhalide, a polymeric material selected from the class consisting ofgelatin, polyvinyl alcohol, water-soluble partially hydrolyzed polyvinylacetals and esters, and water-soluble low viscosity polyvinyl esters.

9. The process of preparing a photographic emulsion which comprisespreparing an aqueous dispersion of a silver halide of small particlesize by conducting in water a double decomposition reaction between awatersoluble halide and a water-soluble silver salt in the presence offrom 0.001% to 0.1%, based on the weight of the total reaction mixture,of a dispersant which is a copolymer consisting of the repeating unitwherein Z is a bivalent alkylene radical of from 2 to 4 carbon atoms,and subsequently adding thereto, as a protective carrier for the silverhalide, a polymeric material selected from the class consisting ofgelatin, polyvinyl alcohol, water-soluble partially hydrolyzed polyvinylacetals and esters, and water-soluble low viscosity polyvinyl esters.

10. The process of preparing a photographic emulsion which comprisespreparing an aqueous dispersion of silver bromide of small particle sizeby contacting in water a double decomposition reaction between sodiumbromide and silver nitrate in the presence of from 0.001% to 0.1%, basedon the total weight of the reaction mixture, of ethylene maleic -acidcopolymer as dispersant, and subsequently adding gelatin thereto as aprotective carrier for the silver bromide.

11. The process of preparing a photographic emulsion which comprisespreparing an aqueous dispersion of silver bromide of small particle sizeby contacting in water a double [decomposition reaction between sodiumbromide and silver nitrate in the presence of from 0.001% to 0.1%, basedon the total weight of the reaction mixture, of the half-amide,half-ammonium salt of isobutylene-maleic acid copolymer, andsubsequently adding galatin thereto as a protective carrier for thesilver bromide.

References Cited in the file of this patent UNITED STATES PATENTS1,981,102 Hagcdorn et a1. Nov. 20, 1934 2,006,002 Schneider June 25,1935 2,276,323 Lowe Mar. 17, 1942 2,378,629 Hanford June 19, 19452,712,003 Bowen June 28, 1955 2,823,200 Longley Feb. 11, 1958 FOREIGNPATENTS 645,625 Germany June 23, 1937 UNITED STATES PATENT OFFICECERTIFICATION OF CORRECTION Patent No. 2,957,767 October 25, 1960Forrest V, Williams It is hereby certified that error appears In theabove numbered patent requiring correction and that the said LettersPatent should read as corrected below.

Column 1 line 24, for "holide" read halide 11116.50, for -"alkali readOalkali column 3, line 25, for "or" read of column 5, lines 19 and 20,for "ethylenemaleic" read ethylene-maleic lines 57 and 58, for"therefrom although" read therefrom. Although column 8, line 17, for"alkali" read -Oalkali column 9 lines 26 and 27, for "alkali" read-O=alkali Signed and sealed this llth day of 311135 71961.

(SEAL) Attest:

ERNEST W. SWIDER I DAVID L. LADD Attesting Officer Commissioner ofPatents

1. A PHOTOGRAPHIC EMULSION COMPRISING A SILVER HALIDE OF SMALL PARTICLESIZE PREPARED BY A DOUBLE DECOMPOSITION REACTION IN WATER BETWEEN AWATER-SOLUBLE HALIDE AND A WATER-SOLUBLE SILVER SALT IN THE PRESENCE OFFROM 0.001% TO 0.1% BASED ON THE WEIGHT OF THE TOTAL REACTION MIXTURE OFA DISPERSANT WHICH IS A COPOLYMER CONSISTING OF THE REPEATING UNIT